(a) Field of the Invention
The present invention relates to a light source integrated with a modulator and a module for optical communication, and especially to the structure for the light source integrated with the modulator and suitable for a light source for long distance trunk used in wavelength division multiplexing (WDM).
(b) Description of the Related Art
As a light source for long distance trunk used in WDM, a module is put into practice which includes a distributed feedback semiconductor laser (DFB-LD) and a field absorbing modulator (MOD) on a single semiconductor substrate. This type of light source can realizes the transmission speed as high as 2.5 Gb/s and has been vigorously researched.
A resistance to distribution or an ability of transmitting long-distance signals in the DFB-LD is the most important parameter exhibiting element performance. The selection of products having no defect therein is currently conducted by measuring the resistance to distribution or the power penalty after the transmission of each element. It is an important subject to increase the yield of the selection for decreasing the cost of the light source.
Murotani et al. reported the dependency of the single mode yield of the DFB/MOD module having an asymmetrical xcex/4 phase shifter (a structure obtained by moving the xcex/4 phase shift region of the diffraction grating from the central position of the DFB-LD, hereinafter referred to as xe2x80x9casymmetrical xcex/4-D-FB/MODxe2x80x9d) on a xcexaL value of a diffraction grating in the DFB-LD in the Electronics Society Meeting (C-4-15, p242) held in September 1999 by the Institute of Electronics, Information and Communication Engineers.
As shown in FIG. 1, the single mode yield rapidly decreases with the increase of the xcexaL in the asymmetrical xcex/4-DFB/MOD module. Conventional publications including the above publication describe neither the yield of the power source integrated with the modulator separately from the single mode yield, nor the optimum structure regarding determination of the facet reflection rate and of the xcexaL. The optimum structure has not been conventionally established by considering the dependency of the diffraction grating on the xcexaL value and the dependency of the modulator on the facet reflection rate.
In view of the foregoing, an object of the present invention is to increase the yield of obtaining DFB/MOD module having no defect by considering the dependency of the xcexaL value of the diffraction grating on the single mode yield and the transmission yield and the dependency of the modulator on the facet reflection rate.
The present invention provides an optical device including: a substrate; a distributed feedback (DFB) semiconductor laser formed on the substrate and including a diffraction grating having an asymmetrical xcex/4 phase shift region, the diffraction grating extending along an optical axis of the DFB semiconductor laser; and a field absorbing modulator integrated with the DFB semiconductor laser on the substrate for modulating a light wave emitted from the DFB semiconductor laser, the modulator having a facet reflection rate between 0.01 and 0.02% at an output end thereof, the diffraction grating having a xcexaL value between 1.4 and 1.7.
In another aspect of the present invention, the facet reflection rate of the output end of the modulator is in a range between 0.02 and 0.03%, and the xcexaL value of the diffraction grating is between 1.2 and 1.3.
In a further aspect of the present invention, the facet reflection rate of the output end of the modulator is in a range between 0.03 and 0.05%, and the xcexaL value of the diffraction grating is between 1.3 and 1-4.
In a still further aspect of the present invention, the facet reflection rate of the output end of the modulator is in a range between 0.05 and 0.1%, and the xcexaL value of the diffraction grating is between 1.4 and 1.7.
In accordance with the present invention, the proper combinations of the xcexaL value of the diffraction grating and the reflection rate of the output facet of the modulator of the DFB semiconductor laser can fabricate the source for integrating the modulator having the excellent quality with a higher yield and lower cost.
The above and other objects, features and advantages of the present invention will be more apparent from the following description.